Polythioaminals are a class of polymers with a variety of uses, including drug delivery. Some polythioaminals are polymers that have the general structure:
where R1 and R2 are organic or hetero-organic species. It has been shown that polythioaminals having the above structure may be synthesized by reacting an N-substituted hexahydrotriazine with a dithiol, as follows:
Subsequent reactions may replace the hydrogen atoms at the end of the thioaminal polymer with the X and Z groups above. These polymers feature dynamic covalent bonding, undergoing reversible bond breakage and reformation. The dynamic covalent character of these polymers is chemosensitive, and may be triggered by the presence of thiols.
One method to tune the physical and/or mechanical properties of these polymers is to increase the molecular weight of the polymers. Whether a polythioaminal can achieve a high molecular weight during polymerization is affected by stoichiometric ratios of starting materials, as described by the Carothers equation. The Carothers equation states that the degree of polymerization of a monomer (into a polymer) is equal to 1/(1−p), where p is the extent of conversion of a monomer. If one monomer is present in stoichiometric excess, then the equation becomes (1+r)/(1+r−2rp), where r is the stoichiometric ratio of reactants, the excess reactant is conventionally the denominator so that r<1. If neither monomer is in excess, then r=1 and the equation reduces to the equimolar case above. Small changes in the stoichiometry of one of the polymerization reactants may significantly affect the molecular weight of a synthesized polymer. Molecular weight of polythioaminals is also limited by a byproduct amine formed during polythioaminal formation. Each carbon atom of a hexahydrotriazine is electrophilic and reacts with the nucleophilic thiols (such as the dithiol shown in the scheme above). As such, this reaction yields the primary amine byproduct. If this primary amine byproduct is not efficiently removed, formation of high molecular weight polythioaminals is very limited. Nonetheless, linear polythioaminals typically do not exhibit attractive physical properties, e.g. they are viscous liquids.
Physical and mechanical properties of polythioaminals may also be tuned by polymerizing multiple polymers from a core compound using multi-functional thiols (at the polymer termini shown above) that may enhance the rigidity (modulus) and dimensional stability (under stress). However, the use of multi-functional thiols reduces the molecular weight of the linear polythioaminals as the crosslink density is increased. These lower molecular weight polythioaminals are typically mechanically weak.
If, however, crosslinking moieties could be included along a polythioaminal backbone without reducing the molecular weight of the polythioaminals, one could more readily achieve tailorable mechanical properties of polythioaminals (such as a desirable modulus for certain applications) and may provide access to elastomeric materials.
Therefore, there is a need in the art for new polythioaminals with improved and tailorable physical and mechanical properties.